The major economic concept that John Maynard Keynes advocated during the Great Depression was government intervention in the economy. In response to the severe economic downturn in the 1930s, Keynes proposed that during periods of low demand, like unemployment, governments should increase public spending and lower taxes to stimulate economic activity. His ideas, articulated in "The General Theory of Employment, Interest, and Money," emphasized the importance of aggregate demand in driving economic growth. Keynesian economics significantly influenced economic policy, leading to a shift towards more active government involvement in managing economic cycles and fostering recovery in subsequent decades.